Sam Altman Testifies His Stake in Helion Energy Exceeds $1.6 Billion

OpenAI CEO Sam Altman testified in court on Tuesday that his stake in nuclear fusion power startup Helion Energy has grown to more than $1.6 billion as of the end of 2025. The value represents a substantial increase from the $375 million he invested in the company in November 2021 when he led its Series E funding round.

Altman said he first invested in Helion in 2015 and has participated in multiple funding rounds since then. He estimated that he owns around a third of the company and holds warrants to purchase additional shares. Helion is currently in discussions with OpenAI on a power purchasing agreement.

Court exhibits shown during the trial listed several other holdings. These included a $632 million stake in fintech company Stripe, a $258 million position in antiaging research firm Retro Biosciences, a $19 million stake in pharmaceutical company Formation Bio, formerly known as Trialspark, and a $3 million investment in AI chipmaker Cerebras.

The disclosures emerged in a trial brought by Elon Musk over OpenAI's shift toward for-profit operations. Musk, who helped start the organization as a nonprofit in 2015, has argued that its founders have diverted resources for personal gain through partnerships including with Microsoft.

During testimony, Altman was questioned about potential conflicts of interest involving the planned power deal between Helion and OpenAI. He stated that he spoke with people at OpenAI about the deal but recused himself from decisions on both sides. He added that he did not sign the agreement and stepped down as Helion's board chair in March 2026.

Altman also addressed his investments in Reddit, where he once served briefly as interim CEO. He confirmed controlling approximately $300 million in Reddit shares around the time of its March 2024 initial public offering. The discussion turned to a content licensing deal between Reddit and OpenAI announced shortly afterward, which he acknowledged presented a conflict that he addressed through a recusal process at his company.

The trial has brought attention to Altman's personal finances. While he is a billionaire, his wealth does not derive from equity in OpenAI itself. The exhibits provided an unusually detailed view of his investment portfolio across multiple startups.

The document Altman signed on April 24 listed values for his positions in various companies. These stakes reflect investments made over several years in sectors including financial technology, biotechnology, nuclear energy and artificial intelligence hardware.

Cerebras, one of the smaller positions disclosed, is preparing for an initial public offering. Formation Bio has rebranded from its previous name as it works on pharmaceutical development. The full scope of Altman's investments remains private outside the context of the court proceedings.